1. Field of Invention
The present invention relates to a voltage regulation circuit, and more particularly to a dual loop voltage regulation circuit of power supply chip.
2. Related Art
Generally, a conventional system of a voltage regulation circuit of current source and current sink is a single close loop system. Due to the use of the single close loop system, there is a need for a relatively large offset voltage to control the output of one of the amplifiers to be high potential, resulting in an overtime of the non-overlap, which makes the output voltage dithering easily. Referring to FIG. 1, a circuit diagram of a conventional voltage regulation circuit is illustrated, when an output voltage (V0*R2)/(R1+R2) is lower or higher than a reference voltage signal Vref, the output of one of the operational amplifiers OP is high potential, and a transistor switch (NMOS) is on to provide a current output. However, both of the operational amplifiers OP have drift voltages, so it is necessary to employ a relatively large reference voltage Vref, namely reference voltage Vref with the addition of an offset voltage Vos, for controlling the output. The control system composed of the voltage (i.e. Vref+Vos) and the offset voltages of the operational amplifiers OP requires the non-overlap time, otherwise shortcut will occur in the transistor switch (NMOS). The non-overlap time will be a long period due to the characteristics of the single close loop, which results in a dithering in the output. Also the relatively large reference voltage Vref will cause an output error, referring to FIG. 4A showing an oscillogram of the voltage and current of the prior art.
Furthermore, with reference to the U.S. Pat. No. 6,433,521, a voltage regulator adapted for an active bus terminator is disclosed, which at least comprises a voltage-regulating end for providing a regulated output voltage; a first operational amplifier with one of the in-phase input ends connected to a reference voltage, for receiving a voltage value acquired by taking away a bias voltage from a reference voltage; a second operational amplifier with one of the inverted input ends connected to the reference voltage, for receiving a voltage value acquired by adding a bias voltage to a reference voltage; a first transistor having a control gate connected to an output end of the first operational amplifier, a source connected to the voltage-regulating end and an inverted input end of the first operational amplifier through a feedback network, which in combination with the first operational amplifier constitutes a voltage regulation circuit having the ability to outflow current; a second transistor, having the control gate connected to an output end of the second operational amplifier, a drain connected to the voltage-regulating end and then to an in-phase input end of the second operational amplifier through the same feedback network, which in combination with the second operational amplifier constitutes a voltage regulation circuit having the ability to sink current, thus avoiding the occurrence of the breakdown current, however, the non-overlap time cannot be reduced, which results in dithering in the output voltage.
Therefore, the problem about how to provide a voltage regulation circuit with relatively stable output is one of the problems to be solved to researchers.